normal fetuses present with a nuchal translucency of greater than 2.5mm. Thus for even greater accuracy of predicting risks, the outcome of the nuchal scan may be combined with the results of simultaneous maternal blood tests. In pregnancies affected by Down syndrome there is a tendency for the levels of human chorionic gonadotropin (hCG) to be increased and pregnancy-associated plasma protein A (PAPP-A) to be decreased. The advantage of nuchal scanning over the previous use of just biochemical blood profiling is mainly the reduction in false positive rates. Nuchal scanning alone detects 62% of all Down syndrome (sensitivity) with a false positive rate of 5.0%; the combination with blood testing gives corresponding values of 73% and 4.7%. In another study values of 79.6% and 2.7% for the combined screening were then improved with the addition of second trimester ultrasound scanning to 89.7% and 4.2% respectively. A further study reported detection of 88% for trisomy 21 (Down syndrome) and 75% for trisomy 18 (Edwards syndrome), with a 3.3% false-positive rate. Finally, using the additional ultrasound feature of an absent nasal bone can further increase detection rates for Down syndrome to more than 95%. When screening is positive, chorionic villus sampling (CVS) or amniocentesis testing is required to confirm the presence of a genetic abnormality. However this procedure carries a small risk of miscarriage so prior screening with low false positive rates are needed to minimize the chance of miscarrying. Development of nuchal translucency The actual anatomic structure whose fluid is seen as translucency is likely the normal skin at the back of the neck, which either may become edematous or in some cases filled with fluid by dilated lymphatic sacs due to altered normal embryological connections. The translucent area measured (the nuchal translucency) is only useful to measure between 11 and 14 weeks of gestation, when the fetal lymphatic system is developing and the peripheral resistance of the placenta is high. After 14 weeks the lymphatic system is likely to have developed sufficiently to drain away any excess fluid, and changes to the placental circulation will result in a drop in peripheral resistance. So after this time any abnormalities causing fluid accumulation may seem to correct themselves and can thus go undetected by nuchal scanning. The buildup in fluid is due to a blockage of fluid in the developing fetal lymphatic system. Progressive increase in the width of the translucent area